Nowadays, R—Fe—B based sintered magnets represented by Nd—Fe—B based sintered magnets have been used in various fields for their high magnetic characteristics. However, an R—Fe—B based sintered magnet contains a highly reactive rare-earth metal: R, and thus is susceptible to oxidization and corrosion in air. Therefore, when such a magnet is used without a surface treatment, corrosion proceeds from the surface due to the presence of small amounts of acids, alkalis, water, etc., whereby rust occurs, causing deterioration or fluctuation in the magnetic characteristics. Further, when such a rusted magnet is incorporated into a device such as a magnetic circuit, the rust may be dispersed and contaminate peripheral parts.
Various methods are known for imparting corrosion resistance to an R—Fe—B based sintered magnet. One of them is a method in which a surface of the magnet is subjected to chemical conversion treatment to form a chemical conversion film. For example, Patent Document 1 describes a method in which a phosphate film is formed as a chemical conversion film on the magnet surface. This method has been widely employed as a simple rust-prevention method for easily imparting necessary corrosion resistance to a magnet.    Patent Document 1: JP-B-4-22008